Sang-Hwan Cho

A high-extraction-efficiency nanopatterned organic light-emitting diode

To improve light extraction from organic light-emitting diodes (OLEDs), we introduced a photonic crystal pattern into the glass substrate of an OLED. The periodic modulation converts the guided waves in the high-refractive-index indium-tin-oxide/organic layers into external leaky waves. We used the finite-difference time-domain method to optimize the structural parameters of the photonic crystal pattern and to analyze the microcavity effect by the metallic cathode of the OLED. With the use of an optimized photonic crystal pattern, an increase of over 80% in the extraction efficiency of the OLED is expected theoretically. An increase in the extraction efficiency of over 50% was achieved experimentally, without detriment to the crucial electrical properties of the OLED.

Planarized SiNx/spin-on-glass photonic crystal organic light-emitting diodes

The light extraction characteristics of low-index spin-on-glass (SOG)-assisted, planarized photonic crystal organic light-emitting diodes (PC OLEDs) are reported. The light extraction efficiencies of planarized two-dimensional (2D) SiNx∕SOG PC OLEDs (type II) and 2D SiNx∕SOG PC OLEDs with an additional high-index SiNx layer (type III) are significantly better under typical operating conditions than those of the first generation of 2D SiO2∕SiNx PC OLEDs (type I). The enhancements in the extraction efficiencies of type-II and type-III PC OLEDs are about 63% and 85%, respectively, with respect to those of conventional OLEDs with indium tin oxide layers of identical thicknesses. These improvements in extraction efficiencies are attributed not only to the liberation of the photons trapped in the high-index guiding layer but also to a reduction in the surface plasmon contribution.

High-power GaN-based blue-violet laser diodes with AlGaN∕GaN multiquantum barriers

AlGaN∕GaN multiquantum barriers (MQBs) were introduced into violet AlInGaN laser diodes with an InGaN multiquantum-well structure, resulting in drastic improvements in lasing performance. Comparing with conventional AlGaN single electron blocking layer (EBL), lower threshold current of 32mA and higher slope efficiency of 1.12W∕A at room temperature has been achieved by using the AlGaN∕GaN multiquantum barrier. This improvement implies that p-type AlGaN∕GaN MQBs are more effective in suppressing the overflow of electrons than p-type AlGaN single EBL. Effective barrier heights of the MQBs should be higher than the single EBL due to the quantum effect of MQBs and the enhancement of p-type doping efficiency. Additionally, the effect of strain on InGaN multiquantum wells from the single EBL can be reduced by using the AlGaN∕GaN MQBs structure.

Enhanced forward efficiency of Y3Al5O12:Ce3+ phosphor from white light-emitting diodes using blue-pass yellow-reflection filter.

This paper reports a simple approach for the design of blue-excitation-light passing and phosphor-yellow-emission-light reflecting dielectric multilayers to recycle the backward emission of Y(3)Al(5)O(12):Ce(3+) (YAG:Ce) yellow phosphors on top of a blue InGaN light-emitting diode (LED) cup. The insertion of modified quarter-wave films of alternate high- and low-refractive index dielectric films (TiO(2)/SiO(2)) into the interface between a YAG:Ce phosphor layer and glass substrate resulted in 1.64 and 1.95 fold increase in efficiency and luminous efficacy of the forward white emission compared with that of a conventional phosphor on top of a blue LED cup with a lower correlated color temperature (< 4000 K).

Far-field radiation of photonic crystal organic light-emitting diode

Utilizing the near- to far-field transformation based on the 3-D finite difference time domain (FDTD) method and Fourier transformation, the far-field profile of a photonic crystal organic light emitting diode is studied to understand the viewing angle dependence. The measured far-field profiles agree well with those of the simulation. The enhancement of the extraction efficiency in excess of 60% is observed for the optimized photonic crystal pattern.

Highly efficient phosphor-converted white organic light-emitting diodes with moderate microcavity and light-recycling filters

We demonstrate the combined effects of a microcavity structure and light-recycling filters (LRFs) on the forward electrical efficiency of phosphor-converted white organic light-emitting diodes (pc-WOLEDs). The introduction of a single pair of low- and high-index layers (SiO(2)/TiO(2)) improves the blue emission from blue OLED and the insertion of blue-passing and yellow-reflecting LRFs enhances the forward yellow emission from the YAG:Ce(3+) phosphors layers. The enhancement of the luminous efficacy of the forward white emission is 1.92 times that of a conventional pc-WOLED with color coordinates of (0.34, 0.34) and a correlated color temperature of about 4800 K.

Lowering Color Temperature of Y3Al5O12 : Ce3 + White Light Emitting Diodes Using Reddish Light-Recycling Filter

A wide variation in correlated color temperature (CCT) in Y 3 Al 5 O 12 :Ce 3+ (YAG:Ce) phosphor-on-cup white light emitting diodes (LEDs) coated with a single phosphor is realized by employing a tunable short wave pass dichroic filter [denoted as a light-recycling filter (LRF)]. An improved balance among blue, green, and red emission spectra is obtained from the reddish LRF-assisted YAG:Ce white LEDs. The lowest CCT of 3720 K in YAG:Ce single-phosphor-converted white LEDs is observed in the reddish LRF-assisted LEDs, where LEDs have an enhanced luminous efficacy (1.58-fold) and reasonable color rendering index (CRI, Ra > 75).

Optical Enhancement in Optoelectronic Devices Using Refractive Index Grading Layers

We enhanced the optical transmittance of a multilayer barrier film by inserting a refractive index grading layer (RIGL). The result indicates that the Fresnel reflection, induced by the difference of refractive indices between Si(x)N(y) and SiO2, is reduced by the RIGL. To eliminate the Fresnel reflection while maintaining high transmittance, the optimized design of grading structures with the RIGL was conducted using an optical simulator. With the RIGL, we achieved averaged transmittance in the visible wavelength region by 89.6%. It is found that the optimized grading structure inserting the multilayer barrier film has a higher optical transmittance (89.6%) in the visible region than that of a no grading sample (82.6%). Furthermore, luminance is enhanced by 14.5% (from 10,190 to 11,670 cd m(-2) at 30 mA cm(-2)) when the grading structure is applied to organic light-emitting diodes. Finally, the results offer new opportunities in development of multilayer barrier films, which assist industrialization of very cost-effective flexible organic electronic devices.

Influence of a two-dimensional SiO2 nanorod structure on the extraction efficiency of ZnS:Mn thin-film electroluminescent devices

To improve light extraction from thin-film electroluminescent (TFEL) devices, we have introduced a two-dimensional (2-D) SiO2 nanorod pattern into the glass substrate of a TFEL. This periodic modulation converts the guided waves in the high refractive index ZnS:Mn phosphor layers into leaked external waves. We used the finite-difference time-domain method to optimize the structural parameters of the 2-D nanorod pattern. Using a nanorod pattern with a depth of 300 nm, a lattice constant of 600 nm, and a radius of 180 nm, the extraction efficiency of the TFEL is theoretically predicted to increase by a factor of more than 4.8. We compare the calculated efficiency and that of the actual TFEL structure, with a view to decreasing diameter of nanorod.

Design and Optical Properties of ZnS:Mn Thin-Film Electroluminescent Devices on 2D SiO2 Corrugated Photonic Crystal Substrates

We studied the characteristics of ZnS:Mn thin-film electroluminescence (TFEL) display devices in which the glass substrate had been modified with a two-dimensional (2D) SiO 2 corrugated photonic crystal (PC) pattern. The finite-difference time-domain (FDTD) method was used to predict the behavior of 2D corrugated PC ZnS:Mn TFEL devices with varying PC structural parameters. Based on the optimal parameters derived from the FDTD calculations, a 2D SiO 2 corrugated PC layer was introduced onto the glass substrate of a typical ZnS:Mn TFEL structure by two-step irradiated hologram lithography and reactive ion etching. Experiments conducted under typical operating conditions showed that incorporation of the 2D corrugated structure improved the light extraction luminance at 40 V above the threshold voltage by over 3.1-fold compared to the conventional ZnS:Mn TFEL device, without noticeable degradation of the electrical characteristics. This improvement originates from the liberation of the photons trapped in the high-index guiding layers.